This invention relates to methods of making tungsten metal powders containing a potassium dopant. More particularly, this invention relates to non-sag tungsten wire for incandescent lamp filaments.
Tungsten wire used in the coiled filaments of incandescent lamps is subject to high mechanical loading and stresses especially in lamps in which the filament operates at temperatures upwards of about 3000xc2x0 C. Pure tungsten wire is not suitable to make filaments for incandescent lamps because under typical operating conditions the individual grains of the filament have a tendency to offset, or slide off (creep or sag) with respect to each other. This causes the filament to sag and short out. A lamp made with pure tungsten filaments will, therefore, fail prematurely.
The beneficial effects of doping to improve the creep resistance of tungsten wire were recognized as early as 1910. In particular, the creep resistance of tungsten wire is known to be improved by doping tungsten blue oxide with potassium-containing compounds to make so-called non-sag (NS) tungsten wire. NS tungsten wire is unique in that it is a microalloy of two very diverse metals, tungsten and potassium. Its creep resistance is due to a minute concentration of potassium of about 75 ppm distributed in the tungsten wire as longitudinal rows of liquid and/or gaseous bubbles. Silicon and aluminum are added with the potassium to serve exclusively as xe2x80x98helpersxe2x80x99 during the reduction and sintering stages. After high-temperature sintering the concentration of Si and Al is reduced to less than 10 ppm each. Neither Si nor Al is known to have any positive role in the final NS tungsten wire.
The long chain of processes in the standard powder metallurgical (P/M) manufacturing of potassium-doped tungsten for incandescent lamps starts with the partial reduction of ammonium paratungstate tetrahydrate (APT), (NH4) 10[H2W12O42]xc2x74H2O, in hydrogen, hydrogen/nitrogen or an inert atmosphere, to produce a tungsten blue oxide (TBO), xNH3xc2x7yH2Oxc2x7WOn, where 0 less than x less than 0.2, 0 less than y less than 0.2, and 2.5 less than n less than 3.0. The TBO is then doped with aqueous solutions 1? containing potassium, silicon and aluminum to a total concentration of about 5000 ppm. The doped TBO is then reduced W with hydrogen to doped tungsten powder. The K, Al, Si-doped tungsten powder, in turn, is washed with hydrofluoric acid, dried, pressed, sintered, rolled or swaged, and drawn. The drawing process can work down the NS tungsten wire to diameters of about 15 xcexcm for use in coiled filaments. The multi-step process finally leads to the outstanding high-temperature creep resistance of NS tungsten wire.
The production of K, Al, Si-doped tungsten powder according to older prior art methods tends to be very inefficient because of the number of steps involved. These processes further produce a contaminated acid waste which must be properly disposed of. More recent methods have made the production of non-sag tungsten wire more efficient by using fewer processing steps and reducing acid waste. For example, U.S. Pat. No. 5,785,731 to Fait et al., which is incorporated herein by reference, discloses a method of making potassium-doped tungsten powder in a one-step reduction of mixed crystals of ammonium potassium paratungstate (AKPT), (NH4)10-xKx[H2W12O42]xc2x74H2O (x=0.04xe2x88x920.4). This method works without doping with Al or Si, and without acid washing. However, two problems remain. First, it is difficult to realize potassium contents higher than 80 ppm and achieve densities of at least 17.2 g/cm3 in sintered tungsten ingots. And second, the method does not prevent or at least remarkably diminish the loss of potassium during reduction of the AKPT.
Another recent method is described in U.S. Pat. No. 6,165,412 to Lunk et al., which is incorporated herein by reference. In that method, a thermally unstable potassium-containing salt or a potassium tungstate is combined with ammonium paratungstate or ammonium metatungstate and then reduced in a single step to form a potassium-doped tungsten powder. Although the single-step reduction offers several advantages, a significant amount of potassium is still lost during reduction.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a method for increasing potassium incorporation in a potassium-doped tungsten powder.
In accordance with an object of the invention, there is provided a method for making a potassium-doped tungsten powder comprising forming a mixture of a tungsten-containing compound, a potassium dopant, and a boron-containing compound; and reducing the mixture to form a potassium-doped tungsten powder.
In another aspect of the invention, there is provided a method for making a potassium-doped tungsten powder comprising forming a mixture of boric acid and ammonium potassium paratungstate, the mixture having a molar ratio of boron to potassium from about 0.6:1 to about 3:1; and reducing the mixture to form a potassium-doped tungsten powder.
In yet another aspect of the invention, there is provided a method for making a potassium-doped tungsten powder comprising forming a mixture of boric acid, a potassium dopant, and a tungsten-containing compound, the potassium dopant being selected from a thermally unstable potassium-containing salt or a potassium tungstate, the tungsten-containing compound being selected from ammonium paratungstate, ammonium metatungstate, or a tungsten oxide, the mixture having a molar ratio of boron to potassium from about 0.6:1 to about 3:1. The mixture is then reduced to form a potassium-doped tungsten powder.